Ultrasonic microprobe hydrophones

Ultrasonic microprobe hydrophones, with element dimensions less than 1 mm, have been constructed in several volume-expander configurations using various piezoelectric materials. The best type, consisting of a single 30×30×15-mil (0.8×0.8×0.4-mm) slab of lead titanate supported by 3-mil (0.08-mm) wires and potted in a ρc polyurethane, was omnidirectional (±1 dB) to 500 kHz and had a sensitivity of −234 dB∥1 V/μPa (±1.5 dB) at the output of a 19.5-dB preamplifier located approximately 1 m from the microprobe. The noise equivalent pressure was approximately 89 dB//1μPa2/Hz at 100 kHz.